Patent Literature 1 describes a conventional mobile crane. In the abstract of the same literature, there is the following description. “The upper swing body is mounted on a lower travelling body through a swing bearing so as to be slewable around a swing center axis. The upper swing body . . . the swing frame (7) having right and left side plates (6R, 6L) . . . ” A parenthesis has been added for reference signs in the description of Patent Literature 1.
In a conventional mobile crane, the axial force on a bearing bolt (bearing-bolt axial force) is locally large. The details of this problem are as follows. FIG. 17 schematically shows the flow of the force that acts on an upper body 1630 or the like of a conventional mobile crane 1001. Upon operation or upon assembly of the mobile crane 1001, a lifting load f1 caused by a suspended load L and a weight 1′2 of a boom 1021 cause a compressive force f3 to act on a portion of a swing frame 1040 on a front side X1 and generates a tension f5 in a raising-lowering rope 1024. The tension f5 causes a force f6 in the direction of an upper side Z1 (vertically upward) and the direction of the front side X1 to act on an end part (lower spreader 1025) of the swing frame 1040 on a rear side X2. As a result, a compressive load f21 acts on a portion of a swing bearing 1005 on the front side X1, and a tensile load f22 acts on a portion of the swing bearing 1005 on the rear side X2. The tensile load f22 is carried by a bearing bolt 1006 shown in FIG. 18. In FIG. 18, only a part of a plurality of the bearing bolts 1006 is denoted by a reference sign. The bearing bolt 1006 is a bolt that fastens the swing bearing 1005 and a bearing seat surface 1050 shown in FIG. 17. As shown in FIG. 18, the position in which a side plate 1042 of the swing frame 1040 and the bearing seat surface 1050 intersect when seen from an up-down direction Z is a side-plate intersecting position 1042a. FIG. 19 shows the relationship of the axial force (bearing-bolt axial force) of the bearing bolt 1006 and an angle θ. As shown in the same figure, the bearing-bolt axial force is locally large in the side-plate intersecting position 1042a (see FIG. 18) and the vicinity thereof (where θ≅±45° in an example shown in FIG. 19). As in the example, with a conventional mobile crane, the bearing-bolt axial force is locally large in the position in which the side plate of the swing frame and the bearing seat surface intersect and the vicinity thereof when seen from the up-down direction.
There are cases where the axial force on the bearing bolt determines the strength of the bearing bolt, and there are cases where the strength of the bearing bolt determines (governs) the lifting capacity and strength of the mobile crane. In such cases, it is necessary to reduce the maximum value of the axial force on the bearing bolt, in order to improve the lifting capacity and strength of the mobile crane.
Generally, by increasing the plate thickness of the bearing seat surface, the stiffness of the bearing seat surface is enhanced, the load distribution of the bearing seat surface is dispersed (localization is suppressed), and the maximum value of the axial force on the bearing bolt is reduced. However, increasing the plate thickness of the bearing seat surface causes a problem of an increase in weight of the mobile crane.